Design a product notifications system
Overview
This question evaluates system design competencies for building a large-scale notifications platform—covering architecture, APIs, data modeling, event pipelines, delivery semantics (retries, deduplication, idempotency), targeting/segmentation, localization, monitoring, and legal compliance such as GDPR/CCPA.
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Aug 14, 2025, 12:00 AM
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System Design
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System Design: Push Notifications and In‑App Messaging Platform
Context
Design a notifications platform for a large consumer mobile app with millions of daily active users. The platform must support both push notifications (iOS/Android) and in‑app messaging, with rich targeting, personalization, and compliance.
Functional Requirements
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User preferences and consent
- Channels: push, in‑app (future‑proof for email/SMS)
- Topics/categories, frequency caps
- Quiet hours and time zone handling
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Segmentation and targeting
- Dynamic segments, real‑time triggers, and scheduled campaigns
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Deduplication
- Across retries and overlapping campaigns; vendor collapse keys
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Rate limiting and throttling
- Per user, per campaign, and global/vendor limits
- Retries, backoff, and idempotency
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Scheduling and batching
- Future send times and batching for efficiency
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A/B experiments
- Randomized assignment, holdouts, metrics
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Localization
- Templates, placeholders, pluralization, fallbacks
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In‑app messaging
- Message inbox, triggers, display rules, and impression tracking
System Requirements
- Describe APIs, data model, event pipeline, delivery workers
- Integrate with APNs (iOS) and FCM (Android)
- Scale to millions of DAUs and bursty traffic
- Multi‑region deployment and CAP trade‑offs
- Monitoring, alerting, and SLOs
- GDPR/CCPA compliance
- Failure handling and graceful degradation
- Capacity estimates and a safe rollout plan
Solution
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